Sexual reproduction involves the fusion of a male and a female gamete. Humans and flowering plants achieve this in very different ways.
The male reproductive system produces sperm in the testes. The female system produces egg cells in the ovaries, releasing about one in each menstrual cycle.
The menstrual cycle prepares the body for pregnancy: the uterus lining thickens, an egg is released, and if it is not fertilised the lining breaks down and is lost during menstruation. In the simplified 28-day model used at school, ovulation happens at around day 14, but in reality cycle length and the timing of ovulation vary.
Fertilisation is the fusion of a sperm nucleus with an egg nucleus, in the oviduct. The fertilised egg divides to form an embryo, which implants in the uterus lining. Gestation in humans lasts about nine months, ending in birth.
The placenta connects the developing fetus to the mother, letting oxygen and nutrients pass in and waste pass out, without the two blood supplies mixing. Because substances cross it, the mother’s lifestyle matters: alcohol, smoking and some drugs can reach and harm the fetus.
A flower’s male parts are the stamens, each an anther on a filament, which produce pollen. The female part is the carpel: stigma, style and ovary.
Pollination is the transfer of pollen from anther to stigma. Insect-pollinated flowers are brightly coloured and scented, produce nectar, and have sticky pollen. Wind-pollinated flowers are small and dull, with anthers and stigmas hanging outside the flower and light, smooth pollen.
After pollination a pollen tube grows down the style. A male gamete nucleus travels down the tube and fuses with the nucleus of the egg cell inside an ovule, which is fertilisation. The ovule becomes a seed and the ovary becomes a fruit.
Seeds are then dispersed away from the parent, so they do not compete with it for light, water and minerals: by wind, by animals eating the fruit or catching burrs on their fur, by water, or by the fruit bursting open.
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